linux-kernel - Re: [RFC PATCH 2/4] sched: Use traced preempt count operations to toggle PREEMPT

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Message-ID: <20150127204239.3d3bc2e4@gandalf.local.home>
Date:	Tue, 27 Jan 2015 20:42:39 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH 2/4] sched: Use traced preempt count operations to
 toggle PREEMPT_ACTIVE

On Wed, 28 Jan 2015 01:24:10 +0100
Frederic Weisbecker <fweisbec@...il.com> wrote:

> d1f74e20b5b064a130cd0743a256c2d3cfe84010 turned PREEMPT_ACTIVE modifiers
> to use raw untraced preempt count operations. Meanwhile this prevents
> from debugging and tracing preemption disabled if we pull that
> responsibility to schedule() callers (see following patches).
> 
> Is there anything we can do about that?
> 

I'm trying to understand how you solved the recursion issue with
preempt_schedule()?

Here's what happens:

preempt_schedule()
  preempt_count_add() -> gets traced by function tracer
     function_trace_call()
         preempt_disable_notrace()
         [...]
         preempt_enable_notrace() -> sees NEED_RESCHED set
             preempt_schedule()
                preempt_count_add() -> gets traced
                   function_trace_call()
                       preempt_disable_notrace()
                       preempt_enable_notrace() -> sees NEED_RESCHED set

                          [etc etc until BOOM!]

Perhaps if you can find a way to clear NEED_RECHED before disabling
preemption, then it would work. But I don't see that in the patch
series.

-- Steve

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